1. Field of the Invention
The present invention relates to a signal converter and a signal conversion method. For example, the present invention is suitably applicable to an up-converter for converting a standard definition (SD) signal such as that of the NTSC system into a high definition (HD) signal such as that of the hi-vision system.
2. Description of the Related Art
In an up-converter of this kind, an HD signal is formed from an SD signal by processing the SD signal by frequency interpolation so that the number of pixels is increased. For example, as shown in FIG. 1, an SD signal represented by larger types of symbols ".largecircle." and ".DELTA." on scanning lines 1A is doubly frequency-interpolated in the horizontal and vertical directions to form an HD signal represented by smaller types of symbols ".largecircle." and ".DELTA.", thereby forming an HD image 1.
As an example of an interpolation method using an up-converter, a method of forming HD pixels at four positions from field data of an SD signal is known. With respect to an SD pixel represented by a symbol ".circleincircle." in FIG. 1, HD pixels are formed at four kinds of positions, i.e., mode-1 to mode-4 positions ,in the vicinity of the SD pixel by interpolation.
As an interpolation filter used for this interpolation, a spatial two-dimensional nonseparable filter 2 shown in FIG. 2 and a horizontal-vertical separable filter 3 shown in FIG. 3 are known.
In the two-dimensional nonseparable filter 2, two-dimensional filter sections 4A to 4D perform interpolation processing independently of each other with respect to HD pixels at mode-1 to mode-4 positions, respectively. The results of interpolation by the two-dimensional filter sections 4A to 4D are combined in series with each other by a selecting section 5 to obtain an HD signal.
The horizontal-vertical separable filter 3 performs processing for modes 1 and 3 by a vertical interpolation filter section 6A and processing for modes 2 and 4 by a vertical interpolation filter section 6B to form data of two HD signal scanning lines. Then the filter 3 interpolates HD pixels at four positions with respect to each scanning line by using horizontal filter sections 7A and 7B and arranges HD pixels in series in a selecting section 8 to form an HD signal.
In the above-described conventional up-converter, the number of pixels is increased but the spatial definition cannot be increased from that defined by an SD signal even if an ideal filter is employed as an interpolation filter. In practice, it is impossible to employ an ideal filter. Therefore, the definition of an HD signal is necessarily lower than that of the SD signal from which the HD signal is formed.
As a solution of such a problem, a classification adaptive processing method has been proposed (in Japanese Patent Laid-Open Publication No. 328185/1993) in which an input SD signal is divided into several classes according to its features and a high-definition HD signal is formed by using prediction coefficients which are prediction data previously formed by learning with respect to the classes.
In the case of forming an HD signal by such a classification adaptive processing method, however, the accuracy of prediction of the HD signal is low if the input signal is not suitably classified according to its features.
Also, if the classification ability at the time of learning is not sufficiently high, it is possible that HD signals, which are to be separated into different classes, are separated into the same class. In such a case, prediction coefficients obtained by learning predict an average value of HD signals differing in characteristics, resulting in a reduction in the ability of restoring the definition of the resulting HD image signal.